Current Medicinal Chemistry - Volume 22, Issue 4, 2015

Biotransformation is one of the key elements of chemically induced toxicity. Although organisms have an intrinsic tendency to diminish the harm posed by chemical exposure with or without structural modification and excretion of the agents (detoxification), this is not always the case; toxification may also occur. The liver has evolved to be the center of biotransformation from the anatomical, physiological and biochemical points of view; it is located alongside the stomach and intestine, it receives more than 25% of the cardiac output and it contains, in general, the richest quantity but also variety of drug metabolizing enzymes. That is why many orally taken drug-induced toxic effects are seen in the liver. Nevertheless, non-hepatic tissues in the organism are also subjected to toxic insult. Although several instances have suggested transport of liver-bioactivated reactive metabolites to the target tissue is responsible, such as monocrotaline-associated lung toxicity, tetraethyl lead- and n-hexane-associated nervous system toxicity and 2-methoxyethanol-associated testis toxicity, etc. [1], the vast majority of data show local bioactivation in the target tissue is responsible for the extrahepatic toxic outcome. The impact of extrahepatic bioactivation and toxicity of drugs can also be seen in cases of drug attrition due to unacceptable toxicity; adverse cardiovascular effects were the foremost reason for drug withdrawals between 1993 and 2006 [2]. On the other hand, the parent drug and/or its stable metabolite( s) may also cause adverse effects such as inhibition of transporters, occlusion of bile secretion (cholestasis) and accumulation in organelles such as mitochondria, causing steatosis in liver and possibly in other organs. However, this review attempts to summarize only extrahepatic bioactivation of drugs/chemicals and their effects at the cellular and tissue level. Specifically, it focuses on the two most perfused organs, lung and heart tissue, as well as thyroid, blood, brain, and skin. Clozapine, a still-in-use drug with severe off-target toxicities (agranulocytosis and cardiovascular toxicity), is investigated in depth and various drugs are reviewed with a special emphasize on the other mentioned organs.

Certain idiosyncratic adverse drug reactions (IADRs) can be triggered by electrophilic protein-reactive metabolites that are formed in the process of drug metabolism. While methodologies (e.g., structural alert concept in drug design, glutathione (GSH) trapping, and protein covalent binding) for examining reactive metabolite (RM) formation are available, predicting the IADR potential applying these parameters remains a significant challenge. The present work examines toxicity trends associated with the aniline structural alert in the top 200 prescribed drugs of 2011 and recently approved (2009-2013) small molecule drugs, in relation with 30 aniline-based drugs withdrawn from commercial use or associated with a black box warning for IADRs. The aniline sub-structure was found in several drugs from the toxic, mostprescribed, and recently approved category. RMs resulting from the bioactivation of the aniline alert was also noted in the three categories chosen for comparison. A major discriminator between the toxic drugs and the majority of drugs in the most-prescribed list, however, was the daily dose – drugs most frequented associated with IADRs were the ones with higher daily doses (exceeding hundreds of milligrams). A greater tolerance for IADRs was also noted with certain drugs intended to treat rare, unmet medical needs (e.g., cancer). Overall, the analysis suggests that optimization of pharmacologic potency and pharmacokinetics that would lead to a lower daily dose, and therefore, a lower body burden of parent drug/metabolites, should be taken into consideration in drug discovery.

Over the past decades, it has become abundantly clear that enzymes evolved to detoxify and eliminate foreign chemicals from the body, occasionally generate highly reactive metabolites which have toxicological implications. To decrease the probability of late clinical failure or market withdrawal, there has been an increased prioritization on understanding key metabolic processes that might cause drug interactions or toxicities. Significant advances have been made in the detection of reactive metabolites and in understanding the structure activity relationship. It is now widely accepted that compounds with certain functional groups such as anilines, quinones, hydrazines, thiophenes, furans, acylpropionic acids, and alkynes have a much greater associated risk towards formation of reactive metabolites than compounds that do not contain such “structural alerts”. Detection of reactive metabolites is usually done with in vitro assays, which have become more sensitive with advances in mass spectrometry. As an increasingly large number of compounds that form reactive metabolites have been identified, much of the focus has shifted from detection to evaluation of toxicological implication. While there is a disproportionate number of compounds metabolized to reactive metabolites that are associated with drug-induced hepatotoxicity and serious skin toxicities such as toxic endothelial necrolysis and Steven’s Johnson syndrome, attempts to predict toxicity based on in vitro testing have been discouraging. In this review we attempt to summarize the experimental options available to evaluate reactive metabolites.

It is known that melatonin (MLT) and some of its metabolites act as antioxidants by scavenging free radicals as well as increasing the activity of antioxidant enzymes in the body. MLT is suggested to exert beneficial effects via various mechanisms in the treatment of many diseases, such as cancer, neurodegenerative diseases, epilepsy, diabetes mellitus and obesity. People working in nightshift exhibit decreased MLT levels that are suggested to be related with increased risk of hormone-related diseases. Similarly blind people were found to have increased MLT levels protecting against many diseases. This review briefly summarizes the published reports supporting these beneficial effects of MLT. Furthermore the present review involves recent developments related to the antioxidant effect of remarkable and multi-faceted molecule MLT as well as its metabolites and its synthesized analogues. The role of MLT as an inhibitor of bioactivation reactions is also discussed.

The endocrine system is a major communication system in the body and is involved in maintenance of the reproductive system, fetal development, growth, maturation, energy production, and metabolism,. The endocrine system responds to the needs of an organism by secreting a wide variety of hormones that enable the body to maintain homeostasis, to respond to external stimuli, and to follow various developmental programs. This occurs through complex signalling cascades,with multiple sites at which the signals can be regulated. Endocrine disrupting compounds (EDCs) affect the endocrine system by simulating the action of the naturally produced hormones, by inhibiting the action of natural hormones, by changing the function and synthesis of hormone receptors, or by altering the synthesis, transport, metabolism, and elimination of hormones. It has been established that exposure to environmental EDCs is a risk factor for disruption of reproductive development and oncogenesis in both humans and wildlife. For accurate risk assessment of EDCs, the possibility of bioactivation through biotransformation processes needs to be included since neglecting these mechanisms may lead to undervaluation of adverse effects on human health caused by EDCs and/or their metabolites. This accurate risk assessment should include: (1) possibility of EDCs to be bioactivated into metabolites with enhanced endocrine disruption (ED) effects, and (2) possibility of EDCs to be biotransformed into reactive metabolites that may cause DNA damage. Here, we present an overview of different metabolic enzymes that are involved in the biotransformation of EDCs. In addition, we describe how biotransformation by Cytochromes P450 (CYPs), human estrogen sulfotransferase 1E1 (SULT1E1) and selected other phase II enzymes, can lead to the formation of bioactive metabolites. This review mainly focuses on CYP- and SULT-mediated bioactivation of estrogenic EDCs and summarizes our views on this topic while also showing the importance of including bioactivation and biotransformation processes for improved risk assessment strategies.

Idiosyncratic drug toxicity has led to the market withdrawal of many drugs in the past. Since animal experiments are not predictive of such toxicity, the pharmaceutical industry continues to seek new methodologies for the prevention of such effects. Although the mechanism of idiosyncratic drug toxicity remains unclear, immune reactions are likely involved. Although drugs with low molecular weights are typically not themselves immunogenic, these drugs may become haptens after being converted to chemically reactive metabolites and becoming covalently cross-linked to proteins. Therefore, screening tests to detect chemically reactive metabolites, most typically by trapping with glutathione, are carried out at early stages of drug development. More quantitative methods are used in later stages of drug development; radioassays for covalent binding (using 14Cor 3H-labeled compounds) are most frequently employed. A zone classification system created by combining previous assessment criteria for the chemically reactive metabolites in vitro (<50 pmole/mg-protein) and for the dose levels in vivo (<10 mg/day) could be used for risk assessment of drug candidates. A mechanism for idiosyncratic, drug-induced hepatotoxicity is proposed by analogy to virus-induced hepatitis, where cytotoxic T lymphocytes play an important role; we suggest that idiosyncrasy reflects the involvement of polymorphisms in the human leucocyte antigen-encoding loci. In fact, a strong correlation has been found between of idiosyncratic drug toxicity and specific human leucocyte antigen genotypes. Therefore, screening of patients for gene biomarkers is expected to reduce the clinical risk of idiosyncratic drug toxicity, thereby prolonging the life cycle of otherwise useful drugs.

Drug metabolism can result in the formation of highly reactive metabolites that are known to play a role in toxicity resulting in a significant proportion of attrition during drug development and clinical use. Thus, the earlier such reactivity was detected, the better. This review summarizes our multi-year project, together with pertinent literature, to examine a battery of in vitro tests capable of detecting the formation of reactive metabolites. Principal prerequisites for such tests were delineated: chemicals known/not known to cause tissue injury and produce reactive metabolites, activation system (mainly human-derived), small- and large-molecular targets (small-molecular trappers, peptides, proteins), analytical techniques (mass spectrometry), and cellular toxicity biomarkers. The current status of in vitro tools to detect reactive intermediates is the following: 1. Small-molecular trapping agents such glutathione or cyanide detect the production of reactive species with high sensitivity by proper MS technique. However, it seems that also putative “negatives” give rise to corresponding adducts. 2. Results from peptide and dG (DNA targeting) trapper studies are generally in line with those of small-molecular trappers, although also important differences exist. These two trapping platforms do not overlap. 3. It is anticipated that the in vitro adduct studies could be fully interpreted only in conjunction with toxicity biomarker (such as the Nrf2 pathway) information from whole cells or tissues. However, while there are tools to characterize the chemical liability and there are correlation between individual/integrated endpoints and toxicity, there are still severe gaps in understanding the mechanisms behind the link between reactive metabolites and adverse effects.